The invention relates to a method and a device for the entrained-flow gasification of solid fuels under pressure.
The known methods for the entrained-flow gasification convert pulverized carbonaceous (C-containing) fuels to the gasification products crude gas and slag at temperatures above the flow point of the ash, using gasification agents that predominantly consist of oxygen. In order to melt the ash, very high temperatures are essential in the gasification chamber. The temperatures range from approximately 1400 to 1600° C., depending on the flow point of the ash. At such high temperatures the crude gases and the slag must be drawn off at the outlet from the entrained-flow gasifier. The specific oxygen demand related to the pulverized fuels is higher by approximately 20 to 30% compared with gasification methods that like the fluid-bed methods function at temperatures obtaining outlet temperatures of the crude gases that are lower by approximately 400 to 600 K. High-ash fuels with ash contents of more than approximately 20 mass-%, related to the dry fuel, and having high ash flow points of over approximately 1400° C. cannot be utilized with energetic efficiency nor at economically acceptable conditions in known entrained-flow gasification methods. Furthermore, the entrained-flow gasification has the decisive disadvantage that very high operational and plant efforts are required in order to separate the slag from the crude gases before the further use thereof if, in particular, it is intended to utilize the sensible heat of the hot crude gases that exit the entrained-flow gasifier for waste-heat steam generation. For that the crude gases must be extremely cooled/quenched directly after having left the gasification chamber of the entrained-flow gasifier over a short distance, before they can be allowed to enter the crude gas heat exchangers. Many versions of external quenching of the crude gases have been developed, with the gas quench and the chemical quench having been successful. The use of the gas quench causes losses in usable heat (exergy). The chemical quench where carbon-containing substances are mixed into the hot crude gas flow largely avoids this disadvantage. In continuous operation, however, the chemical quench involves considerable technological effort and serious drawbacks such as tar and soot formation, and the formation of crusts. In order to avoid the high efforts of utilizing the waste heat, alternatively the hot, slag-laden crude gases exiting the entrained-flow gasifier are water-quenched. This is easy and robust but is disadvantageous in that the low-temperature heat developed by the water evaporation can only rarely be utilized in practice.
DE 26 40 180 B discloses the gasification of solid fuels with different grain sizes (from dust to coarse-grained) using a lower fixed bed and an upper dust gasification zone, whereby a fluidized bed is created above the fixed bed and further above a dust gasification is intended to be performed. The solid fuel is to enter the fluidized bed. In the process itself, it is intended to separate the fuel into proportions of different sizes.
Teachings of DE 26 40 180 B are not suitable to achieve a complete gasification, because substantial amounts of ash-containing coke dust enter the product gas and have to be separated therefrom. Re-feeding the coke dust into the process cannot be considered, as ash-containing dust proportions would very soon enrich in the process to such an extent that the process would come to a standstill. Furthermore, other fundamental problems remain unsolved, particularly the adaptation of the process control to the grain size distributions that frequently change by accident and variations of the ash content of both the total ash and the different grain size fractions. The fixed bed, for example, would “grow into” the fluidized bed when the coarse grain proportion of the solid fuels becomes too high and no sufficient amount of the gasification agent can be fed into the fixed bed for balance and fluid dynamic limitations. At least for the reasons mentioned the process proposal according to DE 26 40 180 B has not been successful in technology.
From the disadvantages listed above the problem of the invention derives as to further develop the fundamentals of the process of entrained-flow gasification in order to solve the serious disadvantages of the entrained-flow gasification, particularly allowing high-ash fuels having high ash-flow temperatures to be processed without problems, the crude gases exiting the gasifier to be fed to a waste heat utilization without demanding any external quenching and the produced ashes or slags to be separated to a low plant technological expense from the crude gases and discharged from the entrained-flow gasifier, while it has to be ensured that the used pulverized fuels are practically completely gasified in the entrained-flow gasifier.